Re: [PATCH 19/19] mm: filemap: Avoid unnecessary barries and waitqueue lookups in unlock_page fastpath

[Peter Zijlstra]

On Tue, May 13, 2014 at 08:27:19AM -0700, Paul E. McKenney wrote:
> > Subject: doc: Update wakeup barrier documentation
> > 
> > As per commit e0acd0a68ec7 ("sched: fix the theoretical signal_wake_up()
> > vs schedule() race") both wakeup and schedule now imply a full barrier.
> > 
> > Furthermore, the barrier is unconditional when calling try_to_wake_up()
> > and has been for a fair while.
> > 
> > Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
> > Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> > Cc: David Howells <dhowells@xxxxxxxxxx>
> > Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> 
> Some questions below.
> 
> 							Thanx, Paul
> 
> > ---
> >  Documentation/memory-barriers.txt | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> > index 46412bded104..dae5158c2382 100644
> > --- a/Documentation/memory-barriers.txt
> > +++ b/Documentation/memory-barriers.txt
> > @@ -1881,9 +1881,9 @@ The whole sequence above is available in various canned forms, all of which
> >  	event_indicated = 1;
> >  	wake_up_process(event_daemon);
> > 
> > -A write memory barrier is implied by wake_up() and co. if and only if they wake
> > -something up.  The barrier occurs before the task state is cleared, and so sits
> > -between the STORE to indicate the event and the STORE to set TASK_RUNNING:
> > +A full memory barrier is implied by wake_up() and co. The barrier occurs
> 
> Last I checked, the memory barrier was guaranteed only if a wakeup
> actually occurred.  If there is a sleep-wakeup race, for example,
> between wait_event_interruptible() and wake_up(), then it looks to me
> that the following can happen:
> 
> o	Task A invokes wait_event_interruptible(), waiting for
> 	X==1.
> 
> o	Before Task A gets anywhere, Task B sets Y=1, does
> 	smp_mb(), then sets X=1.
> 
> o	Task B invokes wake_up(), which invokes __wake_up(), which
> 	acquires the wait_queue_head_t's lock and invokes
> 	__wake_up_common(), which sees nothing to wake up.
> 
> o	Task A tests the condition, finds X==1, and returns without
> 	locks, memory barriers, atomic instructions, or anything else
> 	that would guarantee ordering.
> 
> o	Task A then loads from Y.  Because there have been no memory
> 	barriers, it might well see Y==0.
> 
> So what am I missing here?

Ah, that's what was meant :-) The way I read it was that
wake_up_process() would only imply the barrier if the task actually got
a wakeup (ie. the return value is 1).

But yes, this makes a lot more sense. Sorry for the confusion.

> On the wake_up() side, wake_up() calls __wake_up(), which as mentioned
> earlier calls __wake_up_common() under a lock.  This invokes the
> wake-up function stored by the sleeping task, for example,
> autoremove_wake_function(), which calls default_wake_function(),
> which invokes try_to_wake_up(), which does smp_mb__before_spinlock()
> before acquiring the to-be-waked task's PI lock.
> 
> The definition of smp_mb__before_spinlock() is smp_wmb().  There is
> also an smp_rmb() in try_to_wake_up(), which still does not get us
> to a full memory barrier.  It also calls select_task_rq(), which
> does not seem to guarantee any particular memory ordering (but
> I could easily have missed something).  It also calls ttwu_queue(),
> which invokes ttwu_do_activate() under the RQ lock.  I don't see a
> full memory barrier in ttwu_do_activate(), but again could easily
> have missed one.  Ditto for ttwu_stat().

Ah, yes, so I'll defer to Oleg and Linus to explain that one. As per the
name: smp_mb__before_spinlock() should of course imply a full barrier.
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